1. Field of the Invention
The present invention relates generally to a light sensor, and in particular to an ambient light sensor.
2. The Prior Arts
A conventional ambient light sensor has similar sensitivity to the light as human eye. The responsivity spectrum of the ambient light sensor to light of different wavelengths is very close to the CIE standard photopic luminosity function (V(λ)) with a peak at wavelength 555 nm.
In recent years, as displays and mobile communication devices are widely used, the ambient light sensor is attracting more attention. The conventional ambient light sensor is mainly used to detect ambient background light. The detected signals are processed for adjusting brightness and contrast of the display devices. The ambient light sensor not only can provide the best visual perception to the users for relieving the eyestrain but also save power consumption of the display device for prolonging the operation time of the mobile device and extending the operating life of the display device.
Otherwise, color output has become the basic requirement of the display. However, the color screen leads to greater power consumption, which shortens the battery life of the mobile device. Furthermore, due to the users in motion, the amount of ambient light varies, thus it needs to adjust the power of the backlights in the display according to the brightness of surroundings, thereby providing the proper brightness for viewing. If the power of the backlight in the display can be adjusted according to the ambient light detected by the ambient light sensor, the illumination can match with ambient brightness. It not only improves the visual quality, but also greatly reduces the power consumption of mobile phones or PDA's. It is estimated that the power saved is up to 40%, thereby extending the operating life of the mobile device.
Conventional methods for manufacturing the ambient light sensor for detecting the ambient light are usually classified into three categories. The first method uses silicon-based material to manufacture the ambient light sensor, which has the advantage of integrating the ambient light sensor with the post amplifier and the signal processing IC. However, using the silicon-based material results in the limitation of the detection location and the deviation of the detection results due to absorbing the infrared light that is difficult to eliminate. To resolve the problem, one of solutions is using a special filter to filter the infrared light. Another solution is using a pair of sensors to respectively absorb light with different wavelengths and then processing the signals, thereby eliminating the absorbed infrared light. Even though provided with these solutions, the response spectrum of the sensor is still different from that of the human eye. Thus the sensor can't see the way the human eye does.
The second method integrates the ambient light sensor with the display panel. That is, when using the amorphous silicon thin film transistor (a-Si TFT) to manufacture a liquid crystal display panel, the ambient light sensor and the liquid crystal display panel are manufactured at the same time. Or when manufacturing an organic light-emitting diode panel, the material for manufacturing organic light-emitting diode is used to manufacture the ambient light sensor at the same time. However, this method needs to directly change the design of the panel so that it needs to change the manufacturing process of the panel. At present, this method is only used in the handwriting panel, and is still unable to be used in the ambient light sensor.
In order to overcome the disadvantages of the conventional methods for manufacturing the ambient light sensor described above, a third method is proposed. It uses a material without integrating with any silicon chip or panel, and manufactures the ambient light sensor separately. This method uses a material, which absorbs the light with the wavelength range close to the human eye does, to manufacture the light sensor so as to have a spectral sensitivity similar to that of the human eye. In theory, the ambient light sensor manufactured by this method is more stable and balance in detecting the ambient light, and thus different from the ambient light sensor made of the silicon-based material, which has responsivity spectrum for some light different from that of the human eye. Furthermore, the ambient light sensor made by this method is an independent component, which has characteristics very close to that of the human eye, and it can omits the signal calibration and processing. Thus, the light sensor does not need a lot of components, so its manufacturing cost is lower than that of the conventional ambient light sensors. In addition, the panel and the ambient light sensor are manufactured separately, so it does not need to redesign the panel. The design of the whole system is more flexible. Thus, many manufacturers dedicate themselves to this method for manufacturing the ambient light sensor, which is sensitive to wave range of light close to that of the human eye.